1. Field of the Invention
The invention is related generally to the field of reaming tools used to enlarge the diameter of wellbores drilled through the earth beyond the diameter of a drill bit used to initially drill the wellbore through earth.
2. Background Art
Drill bits used to drill wellbores through earth formations typically have a nominal diameter, that is, a diameter of a borehole that will be created when the drill bit is rotated and impressed axially onto the formations. Frequently it is desirable to enlarge the diameter of the borehole beyond the nominal diameter of the drill bit. Specialized drill bits, known as bi-center bits, have been developed to create boreholes having drilled diameters greater than the diameter of an opening through which such bits will pass when they are not rotated. Other tools for enlarging a borehole beyond the nominal diameter of a symmetric bit (one whose drill diameter is substantially the same as its nominal diameter) include reamer wings. Reamer wings are typically assembled to a drilling tool assembly (drill string) at a selected axial position behind (away from the drilling surface) the drill bit. Reamer wings have cutting elements positioned on blades which extend radially outward from the rotational center of the drill string to a greater distance therefrom than the radius of the drill bit. When the reamer wing is rotated, the cutting elements drill the enlarged borehole.
Reamer wings are described for example in U.S. Pat. No. 5,495,899 issued to Pastusek et al, U.S. Pat. No. 5,497,842 issued to Pastusek et al, and U.S. Pat. No. 5,765,653 issued to Doster et al. Reamer wings typically include a tubular housing or body having a number of longitudinally extensive, azimuthally spaced apart, and generally radially-extending blades. The blades having cutting elements on them. The cutting elements are typically polycrystalline diamond compact inserts, carbide inserts or a combination of these. The reamer wings known in the art are susceptible to drilling a borehole in which the surface of the borehole is not smooth and round. Further, the reaming wings known in the art are susceptible to damage to the cutting elements affixed to the blades. Still further, the reamer wings known in the art are typically unable to drill out equipment used to cement a steel a casing in place in the borehole (float equipment) without damage to the cutting elements on the blades.